Recently, in flat panel display devices of mobile phones or personal digital assistances (PDAs), personal computers, televisions, car-mounted navigation display devices and the like, a thin sheet-shaped cover glass is often arranged on the front side of displays so as to cover a wider region than the image display area thereof, for protecting the displays and for improving the beauty thereof.
Such flat panel display devices are required to be lightweight and thinned, and therefore the cover glass to be used for display protection is also required to be thinned.
However, if the thickness of the cover glass is reduced, the strength thereof lowers and the cover glass itself may be broken by dropping, etc. during use or carrying. Thus, there arises a problem that its primary role of protecting the display devices cannot be fulfilled.
Consequently, in already-existing cover glass, glass produced by a float process (hereinafter this may be referred to as a float glass) is chemically strengthened to form a compressive stress layer on the surface thereof to thereby enhance the scratch resistance of the cover glass.
It has been reported that a float glass is warped after chemical strengthening to lose flatness (Patent Documents 1 to 3). It is said that the warpage may be caused by the heterogeneity between the glass surface not in contact with a molten metal such as molten tin during float forming (hereinafter this may be referred to as top surface) and the glass surface being in contact with the molten metal (hereinafter this may be referred to as bottom surface), thereby providing a difference in the degree of chemical strengthening between the two surfaces.
The warpage of the float glass becomes large with increasing the degree of chemical strengthening. Therefore, in the case where surface compressive stress is set to be higher than before, particularly 600 MPa or more, for responding to the requirement for high scratch resistance, the problem of warpage becomes more obvious.
Patent Document 1 discloses a glass strengthening method of forming an SiO2 film on a glass surface and then chemically strengthening it to thereby control the amount of the ions to enter the glass during chemical strengthening. Patent Documents 2 and 3 disclose a method of reducing the warpage after chemical strengthening by controlling the surface compression stress on the top surface side so as to fall within a specific range.
Heretofore, for reducing the problem of warpage, there have been taken a coping method of reducing the strengthening stress caused by chemical strengthening or performing chemical strengthening after removing a surface heterogeneous layer by grinding treatment, polishing treatment or the like of at least one surface of glass.